I. Field of the Invention
The present invention relates to data communication. More particularly, the present invention relates to a novel and improved method and apparatus for performing rate-matching and reverse rate-matching in a CDMA system.
II. Description of the Related Art
A modern day communication system is required to support a variety of applications. One such communication system is a code division multiple access (CDMA) system that supports voice and data communication between users over a terrestrial link. The use of CDMA techniques in a multiple access communication system is disclosed in U.S. Pat. No. 4,901,307, entitled xe2x80x9cSPREAD SPECTRUM MULTIPLE ACCESS COMMUNICATION SYSTEM USING SATELLITE OR TERRESTRIAL REPEATERS,xe2x80x9d and U.S. Pat. No. 5,103,459, entitled xe2x80x9cSYSTEM AND METHOD FOR GENERATING WAVEFORMS IN A CDMA CELLULAR TELEPHONE SYSTEM,xe2x80x9d both assigned to the assignee of the present invention and incorporated herein by reference.
A CDMA system is typically designed to conform to one or more standards. One such first generation standard is the xe2x80x9cTIA/EIA/IS-95 Terminal-Base Station Compatibility Standard for Dual-Mode Wideband Spread Spectrum Cellular System,xe2x80x9d hereinafter referred to as the IS-95 standard and incorporated herein by reference. The IS-95 CDMA systems are able to transmit voice data and (albeit not efficiently) packet data. A newer generation standard that can more efficiently transmit packet data is offered by a consortium named xe2x80x9c3rd Generation Partnership Projectxe2x80x9d (3GPP) and embodied in a set of documents including Document Nos. 3G TS 25.211, 3G TS 25.212, 3G TS 25.213, 3G TS 25.214, and 3G TR 25.926, which are readily available to the public. The 3GPP standard is hereinafter referred to as the W-CDMA standard and incorporated herein by reference.
The W-CDMA standard defines a channel structure capable of supporting a number of users and designed for efficient transmission of packet data. In accordance with the W-CDMA standard, data to be transmitted is processed as one or more xe2x80x9ctransportxe2x80x9d channels at a higher signaling layer. The transport channels support concurrent transmission of different types of services (e.g., voice, video, data, and so on). The transport channels are then mapped to one or more xe2x80x9cphysicalxe2x80x9d channels that are assigned to a user terminal for a communication (e.g., a call).
The W-CDMA standard allows for a great deal of flexibility in the processing of the transport channels. For example, data for a particular transport channel can be coded using a convolutional code, a Turbo code, or not coded at all. Also, the data can be interleaved over one of four different transmission time intervals (TTI) to provide temporal diversity against deleterious path effects (e.g., fading, multipaths, and so on). Different combinations of coding scheme and interleaving interval can be selected to provide improved performance for different types of services.
The W-CDMA standard also allows the bit rate of the transport channels to change from one TTI to the next. At the transmitter unit, rate-matching is performed such that the total number of bits after the rate-matching is equal to the number of channels bits available for transmission. Bits are repeated or punctured (i.e., deleted) to achieve the desired output bit count.
At the receiver unit, complementary signal processing is performed on the received signal. In particular, de-interleaving is performed for each TTI of received symbols, and inverse rate-matching (also referred to herein as xe2x80x9cde-rate-matchingxe2x80x9d) is performed on the de-interleaved symbols. For improved performance, techniques that can be used to efficiently achieve the de-interleaving and de-rate-matching are highly desirable.
The invention provides various techniques to improve the de-interleaving and de-rate-matching at a receiver unit. In accordance with an aspect of the invention, the de-rate-matching is performed on the received symbols zkxe2x80x2 approximately in real time as these symbols are received from a preceding processing element or as they are provided to a subsequent processing element (e.g., using a direct computation to determine the index m of the de-rate-matched symbols xmxe2x80x2). In accordance with another aspect of the invention, the de-interleaving and de-rate-matching are performed concurrently as the symbols zkxe2x80x2 are provided to a buffer such that de-interleaved and de-rate-matched symbols are stored in the buffer. These processing techniques may allow for the use of a smaller buffer to store the de-rate-matched symbols xmxe2x80x2 and may further reduce the processing delays.
An aspect of the invention provides a method for processing symbols received for a (transport) channel in a communication system (e.g., a W-CDMA system). In accordance with the method, a symbol zkxe2x80x2 associated with a first index k is received and a second index n is determined based on the first index k (to achieve de-interleaving of the symbol zkxe2x80x2). A third index m is then determined based on the second index n (to reverse a rate-matching previously performed on the symbol zkxe2x80x2). The received symbol zkxe2x80x2 is then provided to a buffer at a location related to the third index m. The third index m can be determined from the second index n based on a direct computation or an iterative algorithm, both of which are described below. Different direct computations can be used depending on whether bit repetition or puncturing was performed at the transmission source.
Various aspects, embodiments, and features of the invention are described in further detail below.